Barranquilla in Colombia, a tropical city on the Caribbean coast, may hold the answer to one of the deeper mysteries of the Zika epidemic: why has the world’s second-largest outbreak, after Brazil’s, produced so few birth defects?

In Brazil, more than 2,000 babies have been born with microcephaly, abnormally small heads and brain damage caused by the Zika virus. In Colombia, officials had predicted there might be as many as 700 such babies by the end of this year. There have been merely 47.

The gap has been seen all over the Americas. According to the WHO, the United States has 28 cases — almost all linked to women infected elsewhere. Guatemala has 15, and Martinique has 12.

Had the rest of the Americas been as affected as northeastern Brazil, a tidal wave of microcephaly would be washing over the region. Most experts say that will not happen, but they are at a loss as to why.

Discovering what stopped microcephaly in Colombia may help other countries tamp down the epidemic’s worst effects.

There are some obvious differences between Colombia’s epidemic and Brazil’s. The population here is less than a quarter that of Brazil, and almost half of its residents live at higher altitudes, where mosquitoes are rarer.

And Zika circulated silently for much longer in Brazil. The virus arrived there by early 2014, and not in Colombia until late 2015. Having just fought a severe chikungunya epidemic in 2014, Colombia was more ready than Brazil to send forth the anti-mosquito battalions.

But all that does not seem sufficient to explain the disparity. Increasingly, there is evidence for two other possibilities.

Pregnant women here, alerted to the tragedy unfolding in Brazil, may have sought abortions in greater numbers, officials say. Others seem to have heeded the government’s controversial advice to delay pregnancy altogether.

Dr Miguel Parra-Saavedra, the director of maternal-foetal medicine at the Cedifetal Clinic in Barranquilla and one of the country’s leading high-risk pregnancy specialists, is among the experts who suspect many pregnant women in Colombia, alarmed by news reports, sought ultrasounds and aborted deformed foetuses.

Some of his own patients have done so.

Parra-Saavedra heads a study of Zika-related birth defects in cooperation with the Centers for Disease Control and Prevention. In the course of the research thus far, he has diagnosed 13 cases of foetal microcephaly.

Four of the mothers terminated their pregnancies immediately, he said. Another four, and possibly a fifth, sought abortions but were turned down by their health insurance companies.

Only four patients, Parra-Saavedra said, deliberately chose to have their babies.

Among those who tried to have an abortion was Zuleima, a 37-year-old mother of two healthy daughters.

When she and her husband Jaime, 47, an unemployed mine-machinery operator, learnt that their unborn daughter was microcephalic, they requested what is here called “pregnancy interruption”.

Abortion is legal in Colombia to protect a mother’s health, and the health ministry considers a severely deformed baby a threat to maternal mental well-being.

But Zuleima — who asked that the couple’s surnames not be used because some relatives opposed abortion — was already 31 weeks pregnant.

While the abortion law does not specify which week is too late, her insurer balked, she said, saying it needed time to decide whether to pay for the procedure.

“There were papers and papers to fill out, and the company didn’t say no and didn’t say yes,” she recalled. “They said, ‘We’ll call you later’. They never did — and then it was too late. I had to have the baby.”

She spoke as she nursed her newborn daughter, Milagros. Standing behind her, Dr William O. Contreras, a neurosurgeon, said in English that Milagros had no frontal lobes and that the connection between her two brain hemispheres was abnormally small.

“When this happens,” he said, “there is no intelligence, no coordination, no attention, no initiative, no calculation — and no memories at all.”

Dr Fernando Ruiz, Colombia’s vice-minister for public health, also says that it is “very possible” that abortions lowered the microcephaly rate here.

“Colombia has some of the most progressive laws and regulations in South America,” he said in an interview. With gynaecologists alert to the threat, he said, many women had ultrasounds early enough to make decisions.

Even a very small increase in the abortion rate could account for a sharp reduction in microcephaly.

Just 320 legal abortions were officially reported in Colombia in 2011, according to the Guttmacher Institute, a New York-based research organisation supporting abortion rights. Yet the institute estimates that there actually were 400,400 abortions each year in Colombia.

In this country, most abortions are not performed in clinics by vacuum aspiration, but are induced by misoprostol, a drug that causes strong contractions, said Dr Guido Parra Anaya, the director of the Procrear assisted fertility clinic in Barranquilla.

Any doctor can prescribe the drug, and none are legally obligated to report it.

Misoprostol also is commonly given out by illegal providers here, according to the Guttmacher Institute. Frequently, women are told to take the pills and go to a hospital when heavy bleeding starts, as if they had had a miscarriage. Colombian hospitals treat an estimated 93,000 women a year for post-abortion complications.

In July, Dr Martha Lucia Ospina, the director of Colombia’s National Institutes of Health, reported that foetal deaths reported as miscarriages on death certificates had increased by 8 per cent. The numbers have begun returning to normal only recently.

In Brazil, by contrast, abortion is permitted only in cases of rape or incest or to save the mother’s life, and illegal abortions are hard to get because the police, under pressure from evangelical Christians in Brazil’s Congress, began cracking down on clandestine clinics a decade ago.

Also, because the microcephaly surge in Brazil appeared with no warning, even women who might have risked illegal abortions had no time to receive ultrasounds.

In Colombia, women now normally have three ultrasounds during a pregnancy. The increased screening has made for hard choices.

Microcephaly can also be caused by other viruses or genetic mutations, but the Zika virus causes unprecedented levels of brain damage. “In my 22 years as an ultrasound physician, I have never seen microcephaly like this,” Parra-Saavedra said. “The heads are much smaller, to a severe degree.”

Although pockets of dead cells that foreshadow microcephaly may appear earlier, foetal heads do not become unmistakably small until early in the third trimester.

Health insurers, financially struggling here, are reluctant to approve late abortions because they must pay for neonatal intensive care if the child is born alive.

For mothers, the diagnosis is understandably difficult. By the third trimester, foetuses on ultrasounds look much like newborn babies, not like embryos.

The first scans of Kiara Munoz’s son, Juan Diego, were normal. By the time his microcephaly was evident, she was in her seventh month and could see his face clearly.

“The gynaecologist said I could terminate, and I cried,” said Munoz, who is 18 but looks 15. “It was very hard because the baby was so big. My husband and I decided to keep him. I am hoping for a miracle.”

Colombia’s Zika epidemic peaked in February and was declared over in late July. Many women who became pregnant during that time are still due to give birth, so more microcephaly cases may appear.

But they will still be far fewer than originally predicted.

Ruiz said that based on Brazil’s experience, he had expected to see 700 cases of Zika-related microcephaly this year. Now, he expects 100 to 250 at most.

In December, Ruiz asked women in Colombia to delay pregnancies, and he says he believes many did so, although he cannot prove it yet.

A drop in the birthrate would indicate that many women heeded the advice, but the national health statistics office takes 18 months to tally up each year’s birthrate.

In some Latin American countries, suggestions from health ministers that women delay pregnancy met harsh resistance — both from the Roman Catholic Church and from women’s groups complaining that there was too little access to contraception.

El Salvador faced a backlash when it asked women to stop having children for two years. But Colombia’s health ministry asked women to delay for only six to eight months while officials watched how the epidemic unfolded. Some women found that sensible.

As her husband, Gustavo, a police officer, and her young son Sebastian watched, Madis Dominquez, 27, explained how she happened to be getting a four-month ultrasound in September.

She originally planned to become pregnant last December, she said, “but they said, ‘Please wait six months.’ So I waited till May, when they said it looked safe again.”

How many women followed her example will not be known for some time.

The epidemic appears to be winding down in much of Latin America as the hottest, rainiest months end and as more people, having been bitten, develop immunity.

Some experts thought it might have been premature to declare the epidemic over in Colombia, because the virus had infected less than 1 per cent of the population. In French Polynesia, it infected more than 60 per cent before cases disappeared.

A study done by scientists at London’s Imperial College estimated that across Latin America, where the geography is far more diverse than on Pacific islands, it may be two to three years before widespread immunity stops the epidemic.

In places such as Puerto Rico, where the virus arrived relatively late, microcephaly cases are expected to keep increasing. The first live birth of a child with Zika-related microcephaly was reported in September.

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